Various vehicle crash sensors are known which close an electrical circuit to activate an airbag inflator in response to deceleration experienced under vehicle crash conditions. One type of crash sensor comprises a piston which is movable through a passageway. The piston has a rest position in engagement with a piston seat and an actuated position in engagement with electrical contacts. The electrical contacts form part of a circuit for activating an airbag inflator. The piston is biased into the rest position under normal vehicle operating conditions. The sensor operates to close the electrical circuit in response to a decelerating crash pulse of sufficient magnitude and duration to move the piston from the rest position to the actuated position against the bias and against damping forces exerted on the piston by fluid through which the piston moves.
As the piston moves, the fluid is forced to flow around the moving piston through a clearance between the piston and a surrounding passageway wall. Damping forces exerted by the fluid control movement of the piston between the rest position and the actuated position, and affect sensitivity and performance of the sensor.
One such deceleration sensor is shown in U.S. Pat. No. 4,329,549 wherein a piston is biased into the rest position by means of a magnet located behind the piston seat. A crash force moves the piston out of the rest position. As the piston moves, fluid damping forces act on the piston. Fluid damping is controlled by proportioning the diameters of the piston and the surrounding passageway wall to provide a selected clearance of such size a to restrict the flow of fluid past the piston as the piston traverses the passageway. The performance of the sensor in response to a crash pulse therefore depends upon maintenance of manufacturing tolerances between the diameter of the piston and the diameter of the passageway. Such tolerances are known to call for a high degree of precision, and consequently to be difficult and costly to maintain. Furthermore, the fluid damping cannot be adjusted.
U.S. Pat. No. 4,284,863 shows a crash sensor wherein a piston is biased into a rest position in engagement with a piston seat by a spring. As the piston moves away from the piston seat in response to a deceleration pulse, damping fluid flows through a peripheral clearance between the piston and a bore in which the piston moves, or alternately through an orifice communicating the bore with the external atmosphere of the sensor. In either case, the effects of fluid damping depend on precisely machined dimensional tolerances at the clearance between the piston and the bore.
Another type of sensor is shown in U.S. Pat. No. 3,485,973. Performance characteristics of the sensor with respect to deceleration pulses of varying magnitude and duration are selected by means of adjusting the magnetic force biasing the piston toward the rest position. A threaded stop pin is movable against the piston to move the rest position of the piston axially toward and away from a magnet located at the base of the bore in which the piston moves. Movement of the rest position of the piston axially with respect to the magnet affects the threshold level of force required to unseat the piston as well as the duration of a deceleration pulse required to move the piston into the actuated position against the magnetic force.
The prior art thus fails to provide a vehicle crash sensor which is adjustable to yield a desired sensitivity by means of varying the effect of fluid damping without varying the threshold biasing force on the piston.